There are many reasons for one to lose his or her hair.

The number one reason people lose their hair is, of course, based on your genes. Androgenetic Alopecia is hereditary balding / thinning. Unlike the common misconception, this can be inherited from either your mother or father. This hair loss can start as early as your teen years. A table showing the percentage of men in various age groups with hair loss is below.

18-29 years of age: 15 - 30%
30-39 years of age: 35 - 40%
40-49 years of age: 45%
50-59 years of age: 55%
60-69 years of age: 65%

There is no solid cure for this type of hair loss, but you can certainly learn some tricks here in the Hair Topics Education section.

Other secondary reasons for one to lose their hair could include poor hair care. Chemicals can take their toll on your hair. If you have a hairstyle which pulls the hair tight for extended periods, such as a weave, you are putting yourself at risk of losing some hair. Also, if you shampoo, comb, or brush your hair, either violently, or too often, you are at risk for extra hair loss.

Less likely causes for hair loss include: Stress, Childbirth, High Fever, Severe Infection, Severe Flu, Thyroid Disease, Inadequate Protein in Diet, Medications, Cancer Treatments, Birth Control Pills, Low Serum Iron, Low Serum Iron, Fungus Infection (Ringworm) of the Scalp, or Hair Pulling (Trichotillomania).

Other Hair Loss Articles regarding the question: "why is my hair falling out?":

What Are The Common Hair Loss Causes?
One of the most worrying aspects of hair loss treatment is the tendency of so many people to seek solutions without first determining what has caused their loss in the first place.